In recent years, there has been a considerable interest in optical fiber communication systems operating in the long wavelength range. This is of particular interest because of the need to increase the capacity of present wavelength division multiplex (WDM) systems by adding channels at longer wavelengths. For example, it was shown that by utilizing a spectrum range up to 1602 nm only, the number of channels can be more than doubled.
Market trends to longer wavelengths create an immediate need for optical photodetectors operable within an extended wavelength range. Photodetectors are spectrally limited on the long wavelength side by the bandgap of semiconductor material which is used as an active region. As a result, most widely used photodetectors based on InGaAs exhibit a decrease in responsivity as the operating wavelength is extended beyond approximately 1580-1600 nm.
Numerous attempts have been made to extend a wavelength range of photodetectors. One of them is to maintain photodetectors at elevated temperatures to reduce their bandgaps and thus to extend spectral responsivities. This approach has a number of drawbacks, namely the reduction of material bandgap is accompanied by high leakage current, which increases exponentially with temperature, shortened device lifetime and increased thermal loading on the system. In another approach, by applying strain in the active region, the bandgap of the material can be reduced. In the case of the InGaAs telecom photodetector, for example, strain is avoided by growing the active layer such that its crystalline lattice constant is matched to the InP substrate. By adjusting the In.sub.x Ga.sub.1-x As composition x, layers with larger or smaller lattice constant can be grown, which results in compressive or tensile strain when the layers get deposited on InP. However, the thickness of layers that can be grown without introducing substantial crystalline defects is limited to well known values which depend on the magnitude of the strain. Exceeding such thickness limits causes formation of crystalline defects which can degrade the device performance by increasing the leakage current and limiting its lifetime, see e.g. publication by V. S. Ban, A. M Joshi and N. B. Urli "Characterization of process-induced defects in 2.6 .mu.m InGaAs photodiodes", SPIE, Vol. 1985, pp. 234-243, 1993. Noise can also be increased, as shown, e.g. in publication by D. Pogany, S. Ababou, G. Guillot et. al. "Study of RTS Noise and Excess Currents in Lattice-Mismatched InP/InGaAs/InP Photodetector Arrays", Solid State Electronics, Vol. 38, No. 1, pp. 37-49, 1995. For strain values of interest, the thickness of substantially defect free layers is generally not sufficient to attain the required responsivity specifications of the photodetector. When layers exceeding the defect free thickness are grown, they become mismatched with the substrate, and mismatch disclocations as well as crystalline defects are introduced. Though lattice mismatched devices are available on the market, they have prohibitively high leakage currents for telecom receiver application. The examples of lattice mismatched photodetectors along with discussions of the associated problems may be found in the following publications: K. R. Linga, G. H. Olsen, V. S.Ban et.al. "Dark Current Analysis and Characterization of In.sub.x Ga.sub.1-x As/InAs.sub.y P.sub.1-y Graded Photodiodes with x&gt;0.53 for Response to Longer Wavelengths(&gt;1.7 .mu.m)", Journal of Lightwave Technology, Vol. 10, No. 8, pp. 1050-1054, August 1992; R. U. Martinelli, T. J. Zamerowski and P. A. Longway "2.6 .mu.m InGaAs photodiodes", Applied Physics Letters, vol. 53, No. 11, pp. 989-991, September 1988. Other II-VI and III-V compound semiconductors materials are also available on the market, but their process maturity and performance is somewhat inferior to the requirements of fiberoptic system specifications. In one more approach a sandwiched structure of the active region of the photodetector has been proposed, where layers with different strain interleave, thus allowing to improve some other deteriorating characteristics of the detectors. For example, in U.S. Pat. No. 5,608,230 a target is to reduce a relatively large dark current of the detector, while U.S. Pat. No. 5,536,948 aims to mitigate defect propagation from the base layer to the detector elements. U.S. Pat. No. 4,711,857 to Cheng provides a superlattice detector whose wavelength sensitivity is tunable during manufacturing of the device, and U.S. Pat. No. 5,574,289 to Aoki concentrates on the detector suitable for light signals having different polarizations.
As it follows from the above discussion, an extension of a wavelength range is usually achieved at the expense of deterioration of other important parameters of the photodetectors which remains a significant problem in fiber optic systems.
Accordingly, there is a need for development of alternative structures of optical photodetectors which would provide an operation within an extended wavelength range, while maintaining high performance and reliability.